1. Field of the Invention
The present invention relates to a printing system, a method of generating an image, and a recording medium for realizing the method. More specifically the present invention pertains to a printing system that records at least two types of dots having different diameters on a printing object and generates a multi-tone image expressed by recording densities of the at least two types of dots. The present invention also pertains to a method of generating such an image and a recording medium on which programs for realizing this method are recorded.
2. Discussion of the Background
Color printers, in which a plurality of color inks are discharged from a head, are widely used as an output device of a computer that records a multi-color, multi-tone image processed by the computer. Several methods are applicable to print a multi-color, multi-tone image with three color inks, cyan, magenta, and yellow (CMY). One method is a technique adopted in the conventional printers. This technique expresses the tone of a printing image by the density of dots (frequency of appearance of dots per unit area) while fixing the size of dots formed on a sheet of paper by a spout of ink. Another method adjusts the diameter of dots formed on a sheet of paper, in order to vary the density of dots per unit area. Although the advanced processing of the head for forming ink particles has been improving the density of dots formable per predetermined length or the variable range of the dot diameter, the printers have only the limited printing density (resolution) to 300 through 720 dpi and the limited particle diameter to several ten microns. The resolution of printers is significantly lower than the resolution of silver photography, which has reached several thousand dpi on the film.
In ink jet printers, dots recorded on the sheet of paper may be conspicuous at some printing densities. Further reduction of the dot diameter is accordingly required to improve the printing quality. An increase in number of dot-forming elements per color disposed on a print head is also required to improve the printing speed. By way of example, in an ink jet printer having a print head reciprocating relative to the sheet of paper, one proposed technique increases the number of nozzles per color and thereby the number of dots recorded by one feed along the width of the sheet (this feeding direction is hereinafter referred to as the primary scanning direction), in order to raise the printing speed. In this case, a large number of dot-forming elements (ink nozzles) are arranged in a direction perpendicular to the feeding direction of the print head (this direction is hereinafter referred to as the secondary scanning direction).
The increase in number of the dot-forming elements requires increasing the number of driving circuits. In one conventional ink jet printer that forms dots by spout of ink, electricity is supplied to piezoelectric elements that are used as driving elements for spouting ink from nozzles. This mechanism requires a number of piezoelectric elements and driving circuits corresponding to the number of nozzles. For example, 64 piezoelectric elements are required in the case of 64 nozzles. In the printer that carries out printing with four color inks, the total of 64.times.4=256 piezoelectric elements and driving circuits are required. A conventional printing system outputs a signal that determines the on/off condition of each piezoelectric element via an interface circuit IF and generates a timing pulse (dot clock) Sd for supplying electricity to all the 64 piezoelectric elements at each feeding position of the print head, that is, at the printing position, so as to determine formation or non-formation of a dot at the position. Using the driving elements having sufficiently high response like the piezoelectric elements can narrow the pulse width of this dot clock Sd and thereby vary the diameter of dots. It is not required to drive a plurality of dot-forming elements for the respective colors simultaneously. A common signal line can thus be used to output signals from the CPU to the piezoelectric elements on the respective color heads. This simplifies the circuit structure.
In the conventional printing system, the time of supply of electricity to the driving elements, such as piezoelectric elements, depends upon the pulse width of the dot clock Sd. The conventional printing system thus can not form dots of a desired size, for example, large-diametral dots or small-diametral dots, at desired positions in the course of image processing. This causes the poor matching with the desired image processing. The prior art circuit provides two types of dot clocks Sd and selects one of the dot clocks Sd based on the instruction from the CPU. This enables the diameter of dots formed in one primary scan of the print head to be switched between two levels, but equalizes the diameter of ink particles spouted from a plurality of nozzles, that is, the size of dots formed in the primary scan. The requirement on image processing is generally different at the respective dot-forming positions (pixels) corresponding to the plurality of nozzles. This may result in formation of a small-diametral dot at the position where a large-diametral dot is required by the image processing. The conventional structure that can vary the dot diameter thus does not sufficiently exert its effects.
Another possible technique narrows the pulse width of the dot clock Sd and records an image with only small-diametral dots. This technique, however, requires formation of a large number of dots in printing a high-density area and consumes an undesirably long time for printing. The problem is solved by providing circuits for independently driving the respective dot-forming driving elements and varying the dot diameter by every dot. This structure that varies the dot clock Sd independently on every driving element, however, makes the circuit structure and the driving operation extremely complicated.
These problems also arise in the printer that carries out interlace printing. The following describes the outline of interlace printing and its problems. In an ink jet printer with a print head having a plurality of nozzles arranged in the feeding direction of the sheet of paper, the scattered characteristics of individual nozzles and the non-uniform pitch between a plurality of nozzles deteriorate the picture quality of an image printed on a printing medium. The known printing technique to prevent the deterioration of picture quality forms dots on adjoining lines with different nozzles. This is the technique called interlace printing system.
In the interlace printing system, the pitch of nozzles arranged in the secondary scanning direction on a nozzle array of a print head is set to be an integral multiple of the dot pitch corresponding to the printing resolution. In one example, N nozzles are arranged in the secondary scanning direction on the nozzle array, n nozzles out of the N nozzles arranged on the nozzle array are actually driven, and the interval between the nozzles on the nozzle array is set equal to a k-dot pitch. The nozzle pitch k should be a positive integer that is relatively prime to the value n. The print head is relatively fed in the secondary scanning direction by a fixed distance corresponding to the n-dot pitch after every run of the nozzle array on the primary scanning path. The relative feed in the secondary scanning direction is generally realized by feeding the sheet of paper.
In the interlace printing system, adjoining lines in the secondary scanning direction are printed by different nozzles. Even when there are scatters in the characteristics of the respective nozzles and the nozzle pitch, this structure prevents the deterioration of the picture quality of a printing image due to the scatters and thereby gives a high-quality printing image. In the interlace printing system, however, formation of the large-diametral dots and formation of the small-diametral dots are switched only on every path. This system can thus not form dots of a desired size at desired position and causes the poor matching with the desired image processing.